Fix up an unfinished comment
Clean up loader setup for GLX.
Add a testcase for a bug in the public API.
Fix bug in public entrypoint for epoxy_glx_version()

Unfortunately, for GLX 1.4+ entrypoints (just glxGetProcAddress
currently) or extensions, if there isn't a context bound then we don't
have a dpy and screen available to provide useful debug messages.  Oh
well.
Add a failing testcase for the public API of epoxy_has_glx_extension.
Fix a similar bug to HEAD~2, this time in epoxy_has_glx_extension().